This invention relates to a device and method for making plastic articles from resin, and more particularly, to an improved runner block in a mold assembly and a method for making plastic articles from thermoset resin material using the improved runner block in a mold assembly.
Various resins have generally been molded into articles by means of compression, transfer and injection molding processes and devices. Typical resins include thermoset materials, such as, phenolic resins, unsaturated polyester resins, urea resins, alkyd resins, melamine resins, melamine-phenolic resins and epoxy resins.
Generally, when resins are molded by any of the conventional processes and devices, the resins must be subjected to long molding cycles because heating of the resin to its final cure temperature occurs primarily through conductive heat transfer from the heated mold cavity into the resin material. As the heat transfers into the material, the plastic (resin) begins to shrink away from the cavity wall in the mold assembly. When the plastic material shrinks away from the cavity wall, the heat-transfer rate is slowed thereby increasing cure time of the resin.
Various attempts have been made to overcome the foregoing disadvantages, however, existing devices and processes are still deficient for various reasons. In those devices and processes wherein most or all of the elements of the mold assembly including the runners are heated by the application of an external source of heat, shrinkage of the plastic material, that is, the flowable resin melt, away from the runner walls still occurs, thereby resulting in inadequate heating of the resin material. In many cases, attempts have been made to overcome the foregoing disadvantages by providing various complicated designs and mechanisms for sprues or other resin supply means and for gates located between the runners and the mold cavities. However, most of the prior art designs are awkward to use in mold assemblies and/or cause unacceptable pressure differentials and/or cause defects in the molded plastic parts. Furthermore, in most of the existing devices and processes, there is considerable scrap resin material resulting from the runners and sprues, mold erosion, non-uniform curing, poor part density, poor gas removal, very slow cycle times, poor mold replication and the like.
In U.S. Pat. No. 4,126,292, in order to overcome the problem of equal filling velocity of resin in respective cavities in molds having multiple cavities and in order to overcome adverse effects on various characteristics of mold products, such as formation rate of voids, deformation of inserts, adhesive characteristics of resin to inserts, and the like, it is proposed to progressively or continuously reduce the cross-sectional area of runners in the direction farther from the pot, thereby minimizing a drop in fluid velocity of resin in the runner, and to progressively increase the convergent slope of a gate as the gate is spaced farther from the pot so that the pressure drops in a gate portion and a runner portion are equalized for each of the mold cavities, thereby minimizing a difference in fill time. Thus, to achieve improvements in filling mold cavities with resin material, in U.S. Pat. No. 4,126,292, not only is it necessary to provide convergent sloping portions between the inlet and exit of the respective gates in a mold assembly, each of the exits of each of the gates having the same size and cross-sectional configuration, but it is also necessary to provide the convergent sloping portion of each gate with an increasing inclination with an increase in distance of the corresponding cavities from the pot or sprue so that with the increasing inclination and the exits of the same size and cross-sectional configuration, the sum of a pressure drop in the runner connecting the pot or sprue with the inlet of the gate and a pressure drop in each gate is maintained equal for each of the respective cavities.
In U.S. Pat. No. 3,428,289, it appears that there is an attempt to overcome various prior art problems in devices and processes for molding resin materials, especially thermosetting materials, by providing specially-designed gates having a land of varying width to provide a controlled flow of the casting fluid out of a header, the gate running from the header to the mold cavity along substantially the entire length of the mold cavity adjacent the land wherein the gate width varies according to a complex formula. Thus, it appears that U.S. Pat. No. 3,428,289 teaches a complicated gate device and mechanism for overcoming prior art deficiencies in the making of plastic parts from thermosetting materials.
In view of the foregoing discussion, it is apparent that there is a need to provide a simplified, economical device and process for improving the molding of thermosetting resinous materials for the manufacture of plastic articles.